1. Technical Field
The present disclosure generally relates to seismic prospecting and in particular to methods and apparatus for acquiring and processing geophysical information.
2. Background Information
In the oil and gas exploration industry, geophysical tools and techniques are commonly employed in order to identify a subterranean structure having potential hydrocarbon deposits. Seismic vibratory energy sources, or simply seismic vibrators, have been used in the field many years for generating source signals. A seismic vibrator in its simplest form is merely a heavy vehicle that has the ability to shake the ground at a predetermined range of frequencies to impart vibratory seismic signals into the subsurface of the earth over a specified period of time, which allows for an instantaneous energy level less than impulsive generators such as dynamite.
The imparted energy, termed “seismic source signals” herein, travels through the subsurface and reflects some of the energy from subsurface geological structures or layers. The reflected energy is then transmitted back to the earth's surface where it is recorded using an earth motion detector. The recorded data is processed and interpreted to yield information about location and physical properties of subsurface structures and layers.
The seismic source signal is typically a sweep signal, or simply sweep. Sweeps are sinusoidal vibrations and may have duration times on the order of about 5 to longer than 20 seconds depending on the terrain, the subsurface lithology, economic constraints and physical capabilities of the vibrator. The sinusoidal sweep can be increased in frequency overtime, which is called an “upsweep”. The upsweep is the signal used typically in modern seismic exploration. Also, the sinusoidal sweep can be decreased in frequency overtime, which is called a “downsweep”. The end products of the vibrator sweep are waves that propagate through the earth to return information about the subsurface.
The seismic waves travel through the ground and reflect off subterranean formations. Boundaries between formations of differing material, density or structure often reflect seismic waves, and seismic information relating to these waves is collected, processed interpreted to generate a representation or “pictures” of the subsurface. Any number of exploration systems may be used to gather the desired information for processing. Receiver sensors such as velocity geophones, accelerometers and/or hydrophones may be laid out in lines, or optionally towed in the case of hydrophones, for measuring the amplitude of seismic waves due to the seismic source, reflected off subsurface boundaries, and then returning to the deployed sensors. Multiple source point acquisition surveys provide a method of reducing the time to acquire a complete survey area of data. Traditional single source point acquisition acquires one source point of data at an exclusive time. Multiple source point acquisition is used to acquire many source points of data at an exclusive time, providing for faster acquisition of the data over traditional single source methods. In order to separate each source form an acquired multiple source record, contemporary multiple source methods employ methods that emit longer signals or increase the number of vibratory signals emitted in comparison to the tradition single source emission.